1. Field of the Invention
The present invention relates to an electroluminescent element and, particularly, to an electroluminescent element enabling emission of multi-color light.
2. Description of the Related Art
The color of light emitted from an electroluminescent element is yellowish orange color when ZnS (zinc sulfide) is used as host material and Mn (manganese) serving as a luminescent center is added thereto while, on the other hand, it is green color when Tb (terbium) serving as a luminescent center is added thereto.
Unexamined Japanese Patent Application No. H2-112195 describes an electroluminescent element wherein a luminescent layer of ZnS:Mn using ZnS as host material and having Mn, serving as a luminescent center, added thereto and a luminescent layer of ZnS:Tb using ZnS as host material and having Tb, serving as a luminescent center, added thereto are stacked one upon the other. Red-color and green-color filters are provided on the resulting structure, thereby allowing for emission of multi-color light.
In the above-mentioned electroluminescent element, since two luminescent layers are stacked, a drive voltage (luminescence threshold voltage) becomes high if luminescent layers each having a thickness the same as that in the case of a single-layer electroluminescent element are stacked as they are. Accordingly, in order to decrease the luminescence threshold voltage, it is necessary to decrease the thickness of each luminescent layer, which, however, causes the problem that the luminance of light emitted also decreases.
Also, since the use of color filters are needed for color separation, the luminance of emitted light further decreases due to transmission loss caused by the filters.